Our Growing Planetary Family

What most people have known as a nine-planet solar system may soon grow to include 12 planets or more.

So, what's up with that? Is our solar system actually growing new planets?

Well, not exactly. Although it is true that astronomers are constantly discovering new bodies in space—within and beyond our solar system—an official definition of "planet" may allow others to be added to the planetary family.

By August 24, 2006, the International Astronomical Union is expected to come to an agreement on the first official definition of the term, "planet."

If adopted, a "planet" must be at least 500 miles in diameter, orbit the sun, and have a mass at least one-12,000th that of Earth. A planet would also have to be round; roundness shows that the body's gravity is relatively strong.

In previous proposed definitions, Pluto would have lost planet-hood, which to many astronomers, was unacceptable. Under the current proposal, Pluto keeps its title and three other bodies in our solar system would be added: Pluto's moon Charon (actually, not really a moon, as it does not orbit Pluto like Earth's does), the asteroid Ceres, and another body called 2003 UB313, nicknamed Xena.

Up until now, defining planets has a random history. The word "planet" comes from a Greek word meaning "wanderer," and planets became named after familiar gods of the time. Hence, the planet "Jupiter," named after the Roman god of light and sky, and the planet "Mars," named after the Roman god of war.

Technology has improved since Greek and Roman times, and especially over recent decades. With better telescopes, scientists can see ever finer details of "heavenly" bodies. It was not until very recently, for example, that scientists could see that Ceres was mostly round like planets, rather than irregular-shaped like asteroids. Such clearer views have resulted in the even greater need for clearer definitions among astronomers.

For most people, it is hard to imagine just how big the solar system is. So, it sometimes helps to break things down to more familiar scales. For example, the distance from the Earth to the Sun is 149.6 million kilometers (km) or about 93 million miles (mi). To travel that same distance, you would need to circle the Earth's equator 3,733 times. Making the journey by jetliner would take nearly 21,500 days or 59 years of non-stop flying!

For this week's lesson, you will learn more about each of our solar system's traditional nine planets. Along the way, you will compare their sizes and other attributes on a few down-to-earth scales.

Our Solar System's Spread

Since you cannot actually travel to each of the planets, tour a Virtual Solar System. Begin your journey by exploring the Scale Model of the planets. Read the introduction, and then start with the center of our solar system, The Sun.

Then travel from the center outward, stopping at Mercury, and then Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Make sure to note the position of your browser's scrollbar to get an idea of how you have moved in relative distance from one stop to the next and from the Sun.

To get an even better view of the planets, especially in relation to one another, check out Planet 10, an interactive model. Launch Planet 10 to open and navigate the Solar System Fly-through. Start when it is loaded. Your first look at the planetary orbits through the Sun Cam is a side view. Use the toolbar at the top of the window to change the Sun Cam to Top view. Also, click each symbol on the Planet Cam to zoom in on that specific planet. Click a planet's name on the left-side menu to learn fun facts about each on the Information Pages. In orbit view, use the top-left zoom tool to see the orbiting planets closer or farther away.

When you are done, close that window. To learn even more about each planet, use the site's main menu and explore The Planets one-by-one, starting with Mercury.

Now that you have a better idea of how the planets generally compare to one another, let's take a look at some measured data and convert it to more comprehensible earthly terms.

To do that, visit the Datastore. Use the drop-down menu to Choose a planet, and, again, start with Mercury to browse the entire planet list. As you review the data, record in your own notes the Mass and Distance from Sun for each planet.

Use the measurements of mass to mathematically compare each planet's mass with that of Earth. For example, Mercury is just five percent, or one-twentieth, the mass of Earth. That means, about 20 planets with Mercury's mass equal Earth's mass. Set-up and work through each equation, and then write a word sentence that describes your results. If possible, find two everyday objects that represent the same relative mass difference.

Do similarly to compare distances from the Sun. In this case, convert the distance in kilometers to the number of times it would take to travel around the Earth's equator. (One time around is 40,075 km.) Then, calculate the number of years it would take to fly there, if the average jetliner travels at 290 km/hour.

When you are done with the math, take the Quiz to test what you have learned about the planets.

To learn more about each planet's geological features, visit Exploring the Planets and go to Comparing the Planets and check out each planet's fact page.

Planets On Exhibit

Now, join the Cyber Center to use what you have learned about the planets to help the center create educational exhibits for public display. Register as a new user, if you want to return to the center later. Otherwise, skip registration and go straight to Level 1.

Use the side menu to review your Assignment, and then review the Process: Choose Images, Tell Why, Write Labels, and then Make Exhibit. On your Intern's Desktop, you will see a Photo Album, Researcher's Notes, and a place to write Labels.

Browse through the Photo Album and read the Researcher's Notes. Decide how you want to use the images to tell about one planet or compare one planet to another in some way. When you write the labels for each part of the exhibit, include at least one mathematical comparison that you calculated earlier in this lesson.

Print out each exhibit panel and paste on poster board or hang on a wall. If you have time, read the Summary and go through the Evaluation of your exhibit.

Newspaper Activities

Select locations mentioned in Targetnewspaper, either within or outside of the United States. Determine the distance from where you live to that location.

Pick two planets in our solar system and calculate the average distance between them, based on their distances from the Sun. Calculate how many times you would have to travel from home to the distant location in order to travel the same distance between the two planets. Using either the jetliner's average speed (290 km/h) or the speed of some other type of transportation, calculate how long it would take to travel that distance.

Create a multi-panel exhibit using your calculations and other facts you've learned about the two planets you chose for your calculation. Illustrate your mathematical comparisons using descriptive labels, drawings, and bar charts or other types of graphs.